During the third trimester of human pregnancy the concentrations of deoxycorticosterone (DOC) in maternal plasma are 4-50 times those in nonpregnant women and men. It has been suggested that the increased amount of DOC in maternal plasma originates in the fetal compartment. We considered an alternate explanation for the high levels of DOC in plasma or near-term pregnant women, viz., that DOC may be derived in part from 21-hydroxylation of maternal plama progesterone. To test this hyposthesis we measured the fractional conversion of plasma progesterone to DOC from the relationship between the 3H:14C ratio of the infused tracers, [3H]progesterone and [14C]-DOC, and the 3H:14C ratio or urinary 3 alpha,21-dihydroxy-5 beta-pregnan-20-one (tetrahydro-DOC). The fractional conversion of plasma progesterone to DOC ([rho](BU)P-DOC), measured in this manner, was 0.007 +/- 0.001 (mean +/- SEM, n = 26) in the subjects of this study. The values for [rho](BU)P-DOC varied widely among subjects (0.002-0.022) but the range of values for [rho](BU)P-DOC was similar among women pregnant with an anencephalic or dead fetus, nonpregnant and adrenalectomized women, and men. The transfer constant of conversion of progesterone to DOC in plasma, [rho](BB)P-DOC, remained constant in a nonpregnant woman during the infusion of nonradiolabeled progesterone at rates of 0-14 mg/h. Based on the results of these studied, we conclude that DOC is formed by extra-adrenal 21-hydroxylation of plasma progesterone and that the rate of formation of DOC by this pathway is proportional to the concentration of progesterone in plasma.